Direct Sequence Code Division Multi Access (DS-CDMA) has emerged as the preeminent method for sharing spectrum among a plurality of communication channels, e.g., a plurality of wireless devices using a wireless network cell. DS-CDMA has been proven in practice to offer higher data rates for a given bandwidth allocation than competing systems, e.g., Time Division Multi Access (TDMA) or frequency hoping spread spectrum.
DS-CDMA is also a type of spread spectrum signaling method. As opposed to frequency hopping spread spectrum techniques, a DS-CDMA signal uses an entire allocated bandwidth at any given instant.
In the DS-CDMA signaling method a binary data sequence, which is biased so that the two signal states correspond to equal and opposite sign signal levels, is multiplied by a DS-CDMA code that is biased in the same manner, but is characterized by a much higher frequency. For example, every bit cycle of the binary data sequence typically corresponds to from 7 to 127 signal periods of the DS-CDMA code. The signal periods corresponding to each binary value of the DS-CDMA code are referred to as chips periods. The DS-CDMA code can be represented as a vector with one number or element corresponding to each chip period. A pseudo noise number sequence (PN), in which each element is either one or negative one can be used as a DS-CDMA code. The DS-CDMA code is repeatedly multiplied by successive bits of the binary data sequence. Each communication channel can have a unique DS-CDMA code for the purpose of discrimination. For RF transmission, in order to limit the bandwidth utilized, for each chip period, a chip pulse function is generated that has a polarity dictated by the product of the DS-CDMA code value for the chip period, and the binary data sequence value for the chip period. The series of chip pulse functions can be used to modulate a carrier frequency in a binary phase shift key (BPSK) modulator to produce an RF signal for transmission. Other modulation methods and in fact other media can be used for transmitting DS-CDMA signals.
At a receiver an RF to baseband demodulator is used to demodulate the received RF signal. The demodulator ordinarily includes an in-phase (I) channel and a quadrature phase (Q) channel. I and Q outputs of the RF demodulator are filtered by low pass filters to produce I and Q filtered signals. The filtered signals comprise a filtered version of the series of chip pulses used to modulate the carrier. The filtered signals are sampled by an I and Q channel analog to digital converter to obtain a sequence of complex chip values. A despreader then performs vector dot product operations between bit length sub-sequences of the sequences of complex chip values and a locally stored copy of the DS-CDMA code. If in performing the multiplication, the locally stored DS-CDMA code is properly temporally aligned (e.g., aligned at correct bit start points) with the sequence of complex chip values, then the two instances of the DS-CDMA code (the one by which the data sequence is multiplied in the transmitter, and the one by which the sequence of complex chip values is multiplied in the receiver) will multiply out to unity leaving the original binary data sequence. The despreading operation accumulates amplitude over multiple chip periods and can thereby detect a signal which might be close to a noise floor of the transmitter-receiver system.
One problem that effects the signal to noise ratio that is obtained in DS-CDMA communications is frequency drift. Any discrepancy between a carrier frequency of a received signal, and a local oscillator frequency used by the receiver for demodulating the received signal, will lead to a slow rotation of the complex chip values within the period of a bit. The rotation will lower the amplitude of the signal output by the despreader thereby lowering the signal to noise ratio (SNR). The frequency discrepancy can be caused by a number of factors such as manufacturing tolerances on components used in the transmitter's or the receiver's oscillator, or temperature dependent oscillator component characteristics.
In order to reduce frequency discrepancies, expensive quartz crystal based oscillators have been used to generate accurate, and stable frequency signals for transmitters, and receivers.
What is needed is a DS-CDMA system that can use oscillators that exhibit higher variations in output frequency and while attaining high SNR.